Heretofore, integrated circuits (IC) and large scale integrated circuit (LSI) have been protected from malfunctions caused by dirt, dust, heat, moisture or light in an external atmosphere by sealants for protecting them in order for the circuits to be actually used.
In recent years, the sealants have been gradually shifted from sealing with metal materials and ceramic materials to sealing with resins, and at present, epoxy resin sealants are predominantly used.
In particular, in view of balance between costs and physical properties, epoxy resin compositions using a phenol resin as a curing agent are used in large quantities. The sealants using these epoxy resin compositions have problems such as improvement in mechanical properties as well as:                (i) suppression of crack occurrence at the time of reflow soldering, and        (ii) improvement in electrical reliability.        
Generally, the occurrence of cracks at the time of reflow soldering in (i) is assumed to be caused by water in the resin which expands sharply when exposed to high temperatures at the time of reflow soldering. Hence, means for solving the problem is largely focused on control of moisture absorptivity of the resin, and mechanical strength and adhesion of the resin to metal are also involved in the means on the whole.
However, since a reaction between an epoxy group and a hydroxyl group is a reaction which always produces a hydroxyl group as shown by the following reaction formula (1), hydrophilicity becomes higher due to the hydroxyl group, and even if a basic skeleton is rendered hydrophobic, a reduction in the moisture absorptivity as a whole is limited.Reaction Formula (1): (wherein A represents an epoxy residue, and B represents a phenol residue).
As a technique for solving these problems, a technique disclosed in, for example, EP 959088 which uses a phosphine oxide derivative as a curing accelerator has been studied.
Since an epoxy resin composition using the accelerator is cured by use of the phenol resin, moisture absorptivity is almost the same as that when triphenylphosphine or imidazole which is a commonly used accelerator is used. As for improvements in other physical properties, however, a significant improvement in cracking resistance is seen.
Further, its curing behavior has a significantly great merit from an industrial standpoint in that initial curing takes long time and complete curing takes short time.
Meanwhile, with respect to the improvement in electrical reliability in (ii), the following problems exist.
(ii)-1: As a side reaction at the time of curing, epoxy homopolymerization occurs in some portions. As a result, hydroxyl groups of the phenol resin become excessive, so that the composition has poor moisture resistance and electric characteristics. Further, since the epoxy homopolymerized portions and the excessive phenol resin portions exist in addition to an essential epoxy-phenol resin network, the composition also has poor mechanical properties.
(ii)-2: Mainly due to corrosion of metal portions and current leakage of a semiconductor which are caused by incorporation of free ions, halogen ions in particular, the electrical reliability is adversely affected.
Of these, the ion impurities in (ii)-2 are a problem of purification and purity of the epoxy resin in particular and are not intrinsic. As for the problem of (ii)1, modification of the resin and/or control of the side reaction can cause the epoxy resin composition to fully exhibit physical properties inherent in the epoxy resin composition.
These problems indicated by (ii)-1 and (ii)-2 influence a technique to be described hereinafter.
That is, for the purpose of reducing the moisture absorptivity of the resin in the foregoing problem (i), a reaction between an epoxy group and an ester group as disclosed in Japanese Patent Application Laid-Open No. 53327/1987 applied by Nishikubo et al. has been proposed.
In the publication, a quaternary onium salt and a crown ether complex are set forth as preferable catalysts, and in a paper [Addition Reaction of Epoxy Compound with Ester and Its Application to Synthesis of Polymer, Synthetic Organic Chemistry, Vol. 49, pp. 218 to 233 (1991)] by Nishikubo et al., yields when the catalysts are used as unit reactions are specifically described. According to the paper, although the highest yield is 91% which is a yield when tetrabutylammonium chloride is used, the yields are generally low.
It is needless to say that if an ionic compound such as the quaternary onium salt and the crown ether complex remain in a resin used as a sealant for a semiconductor integrated circuit, this means that the ionic impurities described in the foregoing (ii)-1 are added as a curing accelerator, thereby causing undesirable results such as an electrical short circuit as well as corrosion of metal portions in contact with the impurities, and these undesirable results and the corrosion in turn cause serious defects in products.
Meanwhile, in a general addition reaction between an epoxy resin and a phenol resin, a phosphine such as trialkylphosphine or triarylphosphine, imidazole, a tertiary amine or the like is used as a catalyst, and particularly for sealing a semiconductor, imidazole and the phosphines are often used. When these catalysts are used in the reaction between an epoxy group and an ester group, imidazole having reaction activity is liable to cause epoxy homopolymerization which is the foregoing side reaction, and the problem of the foregoing (ii)-2 is remarkable. On the other hand, although the phosphines do not have these problems, they have slow curing speed and provide substantially no cured product.
Therefore, the idea of preparing a curing agent for an epoxy resin by esterifying some or all hydroxyl groups of the phenol resin as the curing agent for the purpose of obtaining lower hygroscopicity has heretofore not been implemented because no effective curing catalyst has been available.
Under such circumstances, in recent years, a phosphazene catalyst for curing an ester group and an epoxy group effectively has been proposed. More specifically, it has been found that the phosphine oxide derivative disclosed in the foregoing EP 959088 is effective for an epoxy-ester curing reaction. Japanese Patent Application Laid-Open No. 80049/2000 discloses its production and high activity in a unit reaction between an epoxy group and an ester group (reaction between monofunctional compounds), and Japanese Patent Application Laid-Open No. 349662/1999 discloses its application to a sealant.
However, it has been found that due to hydrolytic properties of the phosphine oxide derivative, if a cured resin absorbs water with the derivative contained therein and is exposed to high temperatures as in reflow soldering, the derivative is decomposed easily, and electric conductivity increases significantly. That is, the phosphine oxide derivative is not suitable as a sealant in some cases.
Further, a phosphazenium compound which is a curing catalyst is already disclosed in Japanese Patent Application Laid-Open No. 77289/1998 which also discloses that the phosphazenium compound exhibits high activity in a unit reaction between an epoxy group and an ester group.
In consideration of these facts, it may seem to be easy to use the phosphazenium compound together with an epoxy resin having two or more functional epoxy groups and a phenol resin having two or more functional groups or an ester derivative thereof as a thermosetting resin. However, it is generally known that a basic reaction does not necessarily exhibit the same level of reaction activity under a reaction in a polymer.
That is, it must be considered in the case of a three-dimensional crosslinking curing reaction that as three-dimensional crosslinking proceeds, skeletal steric hindrance occurs due to bondings occurring in the vicinity of functional groups and molecules are fixed along with curing, so that the curing reaction may not proceed easily.
When the ionic phosphazenium compound is used as a catalyst, it is conceivable that the compound may not be a favorable catalyst for a sealant with respect to which control of inclusion of ionic substances is generally strict.
Under the circumstances, in the present invention, an epoxy resin composition suitable for use as a sealant, a cured product thereof, and a curing agent composition for the epoxy resin composition.